Glass-ceramic structures and sintered multilayer substrates thereof with circuit patterns of gold, silver or copper

ABSTRACT

Sintered glass-ceramic substrates containing multi-level, interconnected thick-film circuit patterns of highly conductive metals such as gold, silver or copper are provided which can be fired in air (for gold and silver) or in neutral atmospheres (for copper) at temperatures below the melting points of these metals. This has been made possible by the discovery that finely divided powders of certain glasses described herein sinter to essentially zero porosity at temperatures below 1000° C. while simultaneously maturing to glass-ceramics of low dielectric constant, high flexural strength and low thermal expansivity.

FIELD OF THE INVENTION

This invention relates to glass-ceramic structures and, moreparticularly, to thick or thin film or hybrid, inter-connectedmultilayer substrates made of sintered glass-ceramic insulator andconducting patterns made of thick film gold, silver or copper (forelectronic devices). Also, this invention relates to the process andmaterials for producing such substrates starting with certain glasspowders and conductor "inks" or "pastes" containing finely dividedpowders of gold, silver or copper by the so-called "laminated greensheet" technique at firing temperatures not exceeding the melting pointsof the conductor metal employed. The substrates may be designed withtermination pads for attaching semiconductor chips, connector leads,capacitors, resistors, covers, etc. Interconnections between buriedconductor layers can be achieved through the so-called "vias" formed bymetal-paste-filled holes in the individual layers formed prior tolamination which upon sintering will become densely sintered metalinterconnections.

DESCRIPTION OF THE PRIOR ART

The "laminated green sheet" process for fabricating multilayersubstrates of alumina, mullite and other refractory ceramics areadequately described in prior art (such as U.S. Pat. No. 3,423,517 and3,723,176). While the procedures of this invention are similar to thosedescribed in the above patents, important changes are incorporatedtherein to allow for the use of glass powders of this invention.

Alumina (Al₂ O₃), because of its excellent insulating properties,thermal conductivity, stability and strength is generally the materialof choice for substrate fabrication. However, for certain highperformance applications the relatively high dielectric constant,hereinafter designated by the letter "K", of alumina (K Al₂ O₃ ˜10)entails significant signal propogation delays and noise. In addition,the high mating temperatures of commercial aluminas (˜1600° C.) restrictthe choice of co-sinterable conducting metallurgies to refractory metalssuch as tungsten, molybdenum, platinum, palladium or combinations ofthese with each other or with certain other metals and precludes thesole use of good electrical conductors such as gold, silver, or copperbecause these latter will be molten much before the sinteringtemperature of alumina is reached. A further disadvantage of alumina isits relatively high thermal expansion coefficient (˜65-70×10⁻⁷ /°C.)compared to that of silicon semiconductor chips (˜35×10⁻⁷ /°C.) whichmay, in certain cases, result in some design and reliability concerns.

Glass Ceramics

Stookey, in his basic patent U.S. Pat. No. 2,920,971 on glass-ceramics,has described in detail the theoretical concepts and productiontechniques for such products. In brief, glass-ceramics are obtainedthrough the controlled, in-situ, crystallization of a glass body ofproper composition brought about by a two-step heat treatment procedure.The glass composition generally includes substances called nucleatingagents examples of which are TiO₂, ZrO₂, P₂ O₅, SnO₂ and certaincolloidal metal particles. The resultant body is composed of a multitudeof fine grained crystals of substantially uniform size homogeneouslydispersed in a glassy matrix, the crystal phase constituting the majorportion of the body. The high degree of crystallinity, their very smalldimensions and the absence of porosity make these glass-ceramicsgenerally superior in strength to the precursor glasses and otherproperties such as thermal expansivity, chemical durability etc. closelyresemble those of the crystalline phase formed.

The glass-ceramic bodies made in accordance with the above or similarmethods wherein a glass article shaped by conventional glass makingtechniques such as drawing, pressing, blowing etc. of hot, plastic glassmass followed by conversion to the glass-ceramic state by suitable heattreatments will be, hereinafter, referred to as "bulk-crystallized" orsimply as "bulk" glass-ceramics to distinguish them from the sinteredglass-ceramics of this invention.

There have been references to sinterable glass-ceramics in prior art butthese are not suitable for the present application for one reasone oranother. For example, U.S. Pat. No. 3,825,468 refers to sinteredglass-ceramic bodies which are porous in the interior and non-porous inthe exterior surfaces. Such bodies would have low rupture strengths duemainly to the internal porosity, with typical flexural strengths of lessthan 10,000 psi. Furthermore, the final sintering temperatures for theseglass-ceramics would be well in excess of 1000° C. and hence above themelting points of gold, silver and copper. Another example is U.S. Pat.No. 3,450,546 which describes non-porous, transparent, sinteredglass-ceramics produced by sintering in vacuum at temperatures above1200° C. Helgesson (see "Science of Ceramics", pp. 347-361, published bythe British Ceramic Society, 1976) describes the sintering of a glasspowder of composition 53 wt. % SiO₂, 26 wt. % Al₂ O₃ and 21 wt. % MgO.They could obtain dense, corderite glass-ceramics at a sinteringtemperature of about 950° C. provided the glass powder was given a priorchemical treatment in an alkali solution. In the absence of thistreatment, they found that it was not possible to sinter the glasspowder due to premature surface crystallization.

Numerous glass compositions allow sintering to dense bodies attemperatures below 1000° C. but are unsuitable for the purposes of thisinvention owing to the fact the relatively high fluidity (viscosity of10⁵ to 10⁸ poises) at the sintering temperature would result inexcessive movement of the buried conductor patterns and otherwiseprevent the attainment of the rigid tolerances for dimensions anddistortion that have to be met. The rupture strengths of glasses,typically above 10,000 psi, are also much lower than desired for thisapplication. The glasses of the compositions described herein undergocrystallization during the sintering heat treatment forming pervasiverigid networks of micron-sized crystallites which drastically reducesthe overall fluidity of the body thereby enabling greater dimensionaland distortional control. This very crystallization of refractory phasesin the glass during sintering however, can militate against therealization of dense sintering. In the two types of glass-ceramicsdescribed in this invention, certain principles, described hereinafter,have been discovered that enable one to over-come this difficulty.

SUMMARY OF THE INVENTION

Accordingly, the primary object of this invention is to provideglass-ceramic bodies, having low dielectric constants and other usefulproperties for substrate applications, which can be easily obtained bythe sintering of certain glass powders and concurrent conversion intoglass-ceramics at lower temperatures than similar materials known fromprior art.

Another object is to provide materials of lower dielectric constant thanprior inorganic materials used in multilayer substrate applications.

A further object is to provide new glass-ceramic compositions suitablefor the production of such bodies which are characterized as essentiallynon-porous and which possess microstructures consisting of networks offine crystallites with the residual glass and secondary crystallitesoccupying the interstitial spaces of such networks. This uniquemicrostructure imparts to these glass-ceramics rupture strengthssubstantially higher than in sintered glass-ceramics known from priorart.

Another object is to provide multilayer glass-ceramic substrates whichare compatible with thick film circuitry of gold, silver or copper andco-fireable therewith.

Another object is to provide multilayer substrates having thermalexpansion coefficients closely matched to that of silicon semiconductorchips.

Yet another objective is to provide a method for fabricating multilayersubstrates of glass-ceramics with conductor patterns of gold, silver orcopper.

To these and other ends, the invention embodies among its features amethod of making the body by the said method, new compositions formaking the body by the said method, an article comprising the body and amethod of making the article, hereinafter referred to as a "multilayerglass-ceramic substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows typical dilatometric shrinkage curves of β-spodumene glassceramics of this invention.

FIG. 2 is a photomicrograph of a sintered β-spodumene glass ceramic ofthis invention, by scanning electron microscope (SEM) 1000X.

FIG. 3 shows typical dilatometric shrinkage curves for cordieriteglass-ceramics of this invention.

FIG. 4 is a typical photomicrograph of a sintered α-cordieriteglass-ceramic of this invention (SEM 2000X).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Of the two types of glass-ceramics of this invention, one hasβ-spodumene, Li₂ O.Al₂ O₃.4SiO₂ as the principal crystalline phase whilethe other has cordierite, 2MgO.2Al₂ O₃.5SiO₂, as the main crystallinephase. The common feature of these sintered glass-ceramics, apart fromtheir excellent sinterability below 1000° C., is a microstructure thatcan be described as composed of networks that are highly crystalline,the interstices of which are occupied by small amounts of the residualglass and some discrete secondary crystallites. Such microstructuresdiffer from those observed in "bulk" glass-ceramics in that in thelatter the glassy phase forms the matrix or the network with discretecrystallites dispersed in it. We believe that the unique microstructuresobserved in the glass-ceramics of this invention give rise to their highflexural strengths.

The general composition range of the glass-ceramics applicable for thisinvention are given in Table I.

                  TABLE I                                                         ______________________________________                                        COMPOSITION RANGES (WEIGHT PERCENTAGES)                                       β-Spodumene Type                                                                            Cordierite Type                                            ______________________________________                                        SiO.sub.2   65 to 75       48 to 55                                           Al.sub.2 O.sub.3                                                                          12 to 17       18 to 23                                           MgO         0 to 2         18 to 25                                           CaO         0 to 2         --                                                 BaO         (alone or combined)                                               ZnO         0 to 2         0 to 2                                             Li.sub.2 O  3.5 to 11      0 to 1                                             Na.sub.2 O  1.0 to 3.5     --                                                 K.sub.2 O   (alone or combined)                                                                          --                                                 B.sub.2 O.sub.3                                                                           0 to 2.5       0 to 3                                             P.sub.2 O.sub.5                                                                           0 to 2.5       0 to 3                                             TiO.sub.2   0 to 3         0 to 2.5                                           SnO.sub.2                  0 to 2.5   Total not to                            ZrO.sub.2                  0 to 2.5   exceed 5.0                              F           0 to 3         --                                                 ______________________________________                                    

The ranges of constituents of the above glass-ceramics that will yieldsatisfactory materials are determined by several factor. Important amongthese are:

(a) The requirement for the glass-ceramic to sinter to zero apparentporosity at temperatures below 1000° C., and preferably in the vicinityof 950° C.

(b) The requirement for the thermal expansion coefficient, measured inthe temperature range of 20°-300° C., of the glass-ceramic to be in therange of 20 to 40×10⁻⁷ /°C. and preferably to be close to 30×10⁻⁷ /°C.

Sinterable β-Spodumene Glass-Ceramics

SiO₂ and Al₂ O₃ contents greater than the upper limits given in Table Iwould not allow satisfactory sintering to be achieved. Also, the Li₂ Ocontent should not fall below seven percent except when either B₂ O₃ andF act as fluxes and therefore facilitate sintering; they have the addedadvantage of assisting melting and refining of the glasses. The sodiumand potassium oxides are essential constituents since they promote theformation of binary lithium silicates, particularly the metasilicate asa minor crystalline phase which, as discussed below, plays a major rolein the sintering process.

The SiO₂ and Al₂ O₃ contents must also fall within the specified rangesto ensure the development of the desired volume fraction of β-spodumeneto enable the correct thermal expansion coefficient to be attained aswell as to ensure high strength. The P₂ O₅ and TiO₂ are desirablyincluded to promote internal nucleation of the glass grains; inclusionof ZrO₂ also assists internal nucleation.

Specific Compositions

Examples of specific compositions are given in Table II as follows:

                                      TABLE II                                    __________________________________________________________________________    COMPOSITIONS OF β-SPODUMENE GLASS-CERAMICS                               Glass No.                                                                            1    2   3   4    5   6  7                                             __________________________________________________________________________    SiO.sub.2                                                                            71.5 74.9                                                                              71.0                                                                              71.5 71.5                                                                              71.0                                                                             67.8                                          Al.sub.2 O.sub.3                                                                     15.0 7.5 13.0                                                                              13.0 13.0                                                                              15.0                                                                             16.0                                          MgO    --   --  --  --   --  -- 1.5                                           CaO    --   --  --  --   --  -- 4.5                                           BaO    --   2.0 2.0 --   --  -- --                                            ZnO    --   2.1 2.5  2.0 2.0 2.0                                                                              --                                            Li.sub.2 O                                                                           10.0 8.8 8.0 10.0 8.0 8.0                                                                              4.2                                           Na.sub.2 O                                                                           --   --  --  --   --  -- 0.9                                           K.sub.2 O                                                                             2.0 3.1 2.0  2.0 2.0 2.0                                                                              --                                            B.sub.2 O.sub.3                                                                      --   --  --  --   2.0 0.5                                                                              1.8                                           P.sub.2 O.sub.5                                                                       1.5 1.6 1.5  1.5 1.5 1.5                                                                              --                                            TiO.sub.2                                                                            --   --  --  --   --  -- 2.5                                           F      --   --  --  --   --  -- 0.8                                           Sintering                                                                     Temp. (°C.)                                                                   925  850 990 900  855 *  910                                           Thermal                                                                       Exp. Coeff.                                                                   (°C. .sup.1 × 10.sup.7)                                                  32   83  60  43  29  -- 20                                            Modulus of                                                                    Rupture                                                                       (psi)  30,000                                                                             67,000                                                                            10,400                                                                            63,400                                                                             30,000                                                                            -- 25,200                                        K       6.5 --  6.5  6.3 6.4 -- 5.0                                           (permittivity)                                                                __________________________________________________________________________      *did not sinter satisfactorily at temperatures below 1000° C.    

These glasses were melted at temperatures close to 1500° C. frommixtures of suitable raw materials until free from unreacted materialand gas bubbles. The molten glass was quenced by pouring it into coldwater to produce "cullet" suitable for further grinding. The "cullet"was ground to average particle sizes ranging from 3-7 μm and mixed withsuitable organic binders and solvents to obtain a castable slip orslurry from which thin sheets were cast using conventional doctorblading techniques. The bodies were prepared by laminating a desirednumber of these sheets in a laminating press at moderate temperaturesand pressures (e.g. 100° C. and 3000 psi) to obtain a monolithic "green"body. Specimens for the measurement of rupture strength, thermalexpansion coefficient and dielectric constant measurements were formedas above and fired in air using programmed furances. Priorexperimentation had showed that the heating rate should be low, notgreater than 2° C./minute; faster heating rates resulted in incompletebinder burnout. It is also believed that the use of a relatively slowheating rate is advantageous in allowing surface and internal nucleationprocesses to be completed in a controlled manner.

The modulus of rupture of the sintered glass-ceramics was measured in a3-point bending mode on bar specimens and in general the mean of ten(10) determinations was calculated. Thermal expansion coefficients weremeasured in the range from room temperature to 300° C. using two-pointmethod. Dielectric constants were determined at a frequency of 1 MHZ at25° C.

Typical values of these properties are quoted in Table II. In general,the sintering temperatures given are those yielding optimum results butit should be understood that for each material, a sintering range existsusually extending 20° C. above and below the optimum temperature. Theoptimum holding time at the sintering temperature was two hours, thoughtimes ranging from one to five hours also give satisfactory results.

Of the compositions given in Table II, glasses #1, #4, #5, and #7yielded glass-ceramics having properties suitable for the presentmultilayer substrate application. Compositions #3 and #6 failed tosinter satisfactorily below 1000° C. and composition #2, while ityielded a high strength material, resulted in a glass-ceramic of thermalexpansion coefficient outside the desired range.

X-ray diffraction analysis showed that glass ceramics #1, #4 and #5contained β-spodumene as the major phase and a lithium meta anddisilicates as minor phases. Composition #2 contained only a very smallamount of β-spodumene plus a major amount of an unidentified crystalphase. Composition #3 contained β-spodumene as the major phase alongwith minor amounts of lithium metasilicate and disilicate.

On this basis, it is believed that β-spodumene should be present as amajor phase to enable the desired thermal expansion coefficient to beobtained but that binary lithium silicates must also be present as minorphases to promote sintering and densification of the glass-ceramics attemperatures below 1000° C.

Dilatometric shrinkage measurements of green laminates as a functiontemperature illustrate very well the differences between those materialscontaining lithium metasilicate as a minor phase and those that do notcontain this phase. Curve A, FIG. 1, is typical of the former type (e.g.composition 190 1) and Curve B, FIG. 1 of the latter type of material(e.g. glasses #2 and #6). For the satisfactory material (Curve A)densification commences at a temperature of about 580° C. At this stage,the material is still uncrystallized glass. At a temperature of about610° C., however, further densification is arrested owing to the onsetof crystallization. A second densification stage commences at about 910°C. and this proceeds until the glass-ceramic becomes non-porous. For theunsatisfactory materials (Curve B), the second densification stage isabsent at temperatures below 1000° C.

Differential thermal analysis, x-ray diffraction analysis and electronmicroscopic studies have indicated that the commencement of the secondstage of the sintering process corresponds to the liquidus temperatureof lithium metasilicate phase. Some of the metasilicate recrystallizeson cooling the glass-ceramic.

It is believed that the satisfactory sintering of these β-spodumeneglass-ceramics involves the following steps. Initial sintering of theglass powder by viscous flow and diffusion processes, possibly alsoassisted by glass-in-glass phase separation, occurs between 580°-600° C.As a result of surface nucleation processes, the individual glass grainsbecome covered with a layer of lithium metasilicate. Internal nucleationwithin the glass grains follows causing the precipitation of β-spodumenecrystals, whose maximum size will be set by the particle diameter.Further sintering then requires the partial or complete melting of themetasilicate phase which brings about (i) consolidation of theβ-spodumene grains by capillary forces and (ii) neck or bridge formationbetween the β-spodumene particles promoted by the reaction of the moltenmetasilicate with the alumina and silica of the residual glass to formfurther quantities of β-spodumene. At temperatures above the recommendedsintering range, the metasilicate appears to flux the β-spodumenedestroying the inter-particle bridges. The reaction of the lithiummetasilicate with residual glass to form β-spodumene in certain Li₂O--Al₂ O₃ --SiO₂ systems has also been alluded to by Stookey in U.S.Pat. No. 2,971,853.

The microstructure of the glass-ceramics obtained from such a sinteringmechanism consists of cosintered β-spodumene crystals forming rigidskeletal structures with the residual glass occupying the interstitialregions in such a structure. FIG. 2 is an example of such amicrostructure which more resembles those of conventional ceramicsobtained by sintering ceramic powders, such as alumina with minor glassadditions than those of "bulk" glass-ceramics. The absence of acontinuous glassy matrix or network is believed to be the principalfactor governing the high flexural strengths of the present materials.

It is to be understood that as used herein and in the claims, the term"β-spodumene glass" is defined as and restricted to (1) a precursor forβ-spodumene glass ceramic and (2) formed from a batch consisting of, byweight:

    ______________________________________                                        SiO.sub.2                                                                             65 to 75%     LiO.sub.2                                                                              3.5 to 11%                                     Al.sub.2 O.sub.3                                                                      12 to 17%     B.sub.2 O.sub.3                                                                          0 to 2.5%                                    MgO     0 to 2%       P.sub.2 O.sub.5                                                                          0 to 2.5%                                    ZnO     0 to 2%       F          0 to 3%, and                                 ______________________________________                                    

also from 0 to 2% of at least one oxide selected from the groupconsisting of CaO and BaO, and from 1.0 to 3.5% of at least one oxideselected from the group consisting of Na₂ O and K₂ O.

Conversely, as used herein and in the claims, the term "β-spodumeneglass ceramic" is defined as and restricted to a glass ceramic structurecoalesced and crystallized from "β-spodumene glasses" into an articlehaving a microstructure of a pervasive continuous network of 2 to 5 μmcrystallites of β-spodumene with the interstices of said networkoccupied by residual glass having dispersed therein discrete secondary 1to 2 μm crystallites of lithium metasilicate.

Sinterable Cordierite Glass-Ceramics

The overall composition ranges of the cordierite glass-ceramics of thisinvention is given in Table I and specific examples are listed in TableIII.

                                      TABLE III                                   __________________________________________________________________________    CORDIERITE GLASS-CERAMICS                                                             (Weight % Compositions)                                               Glass No.                                                                             8   9   10  11  12  13  14  15  16  17  18  19                        __________________________________________________________________________    MgO     24  24  24.2                                                                              21.5                                                                              22  23  22  23.5                                                                              23.5                                                                              24.0                                                                              24.0                                                                              24.7                      Al.sub.2 O.sub.3                                                                      21  21  21.2                                                                              21  22  22  22  21  21  21.0                                                                              21.84                                                                             21.0                      SiO.sub.2                                                                             53  52  50.6                                                                              52.5                                                                              52.5                                                                              52  52  52.5                                                                              52.5                                                                              52.5                                                                              52.5                                                                              51.8                      P.sub.2 O.sub.5                                                                        2   2   2.0                                                                               2   1.5                                                                               2   2  --  --   1.0                                                                               1.16                                                                              1.0                      Li.sub.2 O                                                                            --   1  --  --  --  --  --  --  --  --  --  --                        B.sub.2 O.sub.3                                                                       --  --   2.0                                                                               1   0.5                                                                               1   1  1    1   0.5                                                                               0.5                                                                               0.5                      ZrO.sub.2                                                                             --  --  --   2   1.5                                                                              --  --  --  --  --  --  --                        ZnO     --  --  --  --  --  --   1  --  --  --  --   1.0                      TiO.sub.2                                                                             --  --  --  --  --  --  --   2  --  --  --  --                        SnO.sub.2                                                                             --  --  --  --  --  --  --  --   2  --  --  --                        Sintering                                                                     Temp. (°C.)                                                                    1050                                                                              960 925 925 950 967 967 967 970 970 972 925                       Thermal                                                                       Expansion                                                                     Coeff.                                                                        (°C. .sup.1 × 10.sup.7)                                                  37  55  30  33  24  23  24  30  33   33  34                           Modulus                                                                       of Rupture                                                                    (psi)   30,500                                                                            42,300                                                                            29,000                                                                            42,300                                                                            82,300                                                                            39,000                                                                            52,000                                                                            39,800                                                                            30,200                                                                            30,200                                                                            30,800                                                                            46,500                    Permittivity (k)                                                                              5.6*                                                                               5.7**   5.3*                                                                              5.4*                                                                              5.7*                                                                              5.6*                                                                              5.6*                                                                              5.7*                                                                              5.5*                     __________________________________________________________________________     *Frequency 1 MHZ                                                              **Frequency 1 KHZ                                                        

The composition limits are set on the one hand by the need to ensurethat cordierite appears as the major crystalline phase in order toachieve desired thermal expansion coefficients and on the other tofacilitate sintering below 1000° C. Reduction of MgO and Al₂ O₃ contentsbelow the specified limits is not permissible for this reason.Excessively high Al₂ O₃ and SiO₂ contents would result in materials notcapable of sintering below 1000° C. MgO contents higher than thespecified maximum could result in the formation of magnesium silicatesin significant amounts causing the thermal expansion coefficients to behigher than desired.

The minor constituents are included to perform important functions. TheP₂ O₅, ZrO₂, TiO₂ and SnO₂ are added to promote nucleation and toregulate the microstructural development. The Li₂ O and B₂ O₃ areincluded as sintering aids; they also serve to regulate the nature ofthe crystalline phase formed. Cordierite can appear in either the μ or αform. Sometimes, mixtures of the two appear in the same glass-ceramic.As will become clear in what follows, it has been discovered that inorder to produce glass-ceramics having stable thermal expansioncoefficients in the desired range as well as lower dielectric constants,it is necessary to develop the cordierite phase predominantly in the αform.

The method of glass preparation, grinding and green body preparation aresimilar to that given for the β-spodumene compositions. The averageparticle size for the glass powder should be in the range of 2 to 7 μmfor good sintering and strength.

Table III gives the optimum sintering temperatures for the cordieriteceramics. It has also been discovered, however, that satisfactorymaterials can be produced at sintering temperatures spanning 80°-100° C.covered by the exothermic peak in the thermograms of the correspondingglasses. For example, composition #10 can be satisfactorily sintered attemperatures within the range of 870° C. to 950° C. and the variation ofthe thermal expansion coefficient for materials sintered within thisrange is only ±1×10⁷ /°C.

The crystalline phases developed in the different glass-ceramics areinfluenced by the minor constituents and in some cases by the sinteringtemperatures employed. Composition #8 forms α-cordierite as a majorphase, together with minor amounts of μ-cordierite. Composition #9contains μ-cordierite as the only crystalline phase. The formation ofμ-cordierite confers to the glass-ceramic a somewhat higher thermalexpansion coefficient as well as an apparent higher dielectricconstants. It is evident that the minor constituent Li₂ O catalyses theformation of μ-form of cordierite. Composition #10 contains onlyα-cordierite due, it is believed, to the presence of boric oxide in theglass composition. Composition #11 contains μ-cordierite as the majorphase along with some α-cordierite. Although the thermal expansioncoefficient of this material for the sintering temperature of 925° C.falls within the desired range of 20 to 50×10⁷ /°C., we have noted thatthe thermal expansion coefficient is dependent on the sinteringtemperature employed. Material sintered at 970° C. has an expansioncoefficient of 36.4×10⁷ /°C. and that sintered at 990° C. has the valueof 40×10⁷ /°C. The enhanced strength for composition #11 is thought toresult from improved crystal nucleation promoted by the inclusion ofZrO₂. This nucleant, however, also promotes the formation of theμ-cordierite phase if its concentration is above a critical limit.Composition #12, containing lower concentrations of B₂ O₃ and ZrO₂compared to composition #11, develops α-cordierite as the major crystalphase together with clinoenstatite as a minor phase. The thermalexpansion coefficient of this material is stable over a wide sinteringtemperature range. For sintering temperatures between 915° C. and 970°C., the expansion coefficient only varied for 23×10⁷ /°C. to 24×10⁷ /°C.The high strength of composition #12 is attributed on the basis ofmicroscopic and x-ray diffraction studies to a high volume fraction ofthe crystalline phase which occurs as a crystalline network having avery small domain size. The formation of clinoenstatite minor phase inthe residual glass during sintering is also likely to have contributedto the high flexural strength of this material.

Investigation has shown that the sintering process for the cordieriteglass-ceramics is fundamentally different from that for the β-spodumeneglass-ceramics. FIG. 3 shows typical shrinkage curves for a materialthat sinters to zero porosity below 1000° C. (Curve A, e.g. composition#10, Table III) and for a material that does not undergo completedensification below 1000° C. (Curve B, e.g. composition #8). It will beseen that unlike the β-spodumene glass-ceramics, the cordieritematerials undergo sintering in a single stage. For materials that aresinterable below 1000° C., we believe that the densification involvespredominantly glass-to-glass coalescence. For example, composition #10can be sintered to negligible porosity at a temperature of 850° C. butexamination of the material fired to this temperature shows very littlecrystallinity in it.

Based on optical and electron microscopic observations and on x-raydiffraction results, it is believed that the sintering process for thesematerials is as follows: After the burn-out of the organic binders,there is no further dimensional change until glass particles begin tocoalesce by viscous and diffusional mechanisms, perhaps also assisted bythe glass-in-glass phase separation observed in this temperature range.Soon thereafter, an interconnecting network of crystallites appearsroughly delineating the prior glass particle boudaries, this leading usto believe that surface nucleation must have occurred on the individualglass grains prior to their coalescence. The formation of these highlycrystalline networks acts to arrest the excessive deformation by viscousflow of the body. Internal nucleation and crystallization within theglassy domains occurs at a slightly higher temperature, this beingpromoted by the added nucleants such as P₂ O₅, ZrO₂, TiO₂ and ZnO. Thismechanism is well illustrated in an experiment in which a bundle offibers glass #10 of each of about 0.2 mm diameter was subjected to thesame thermal cycle as was used for sintering. The glass fibers sinteredtogether at their points of contact but each fiber had developed ahighly crystalline skin to a depth of about 1-2 μm. The interiors of thefibers remained largely glassy. The function of additives such as Li₂ Oand B₂ O₃ may well be to delay the onset of crystallization therebyallowing sintering within desired temperature range.

It is believed that the critical factors that enable the distortion-freesintering to near theoretical densities of these glass-ceramics are thefollowing: (i) the absence of a well-defined nucleation hold on the wayto the sintering temperature which prevents internal nucleation and ofcrystallization prior to the completion of glass-to-glass sintering,(ii) the relative ease of surface nucleation compared to bulk nucleationin these glasses, such nucleation occurring despite the factor (i) aboveprior to glass-to-glass sintering, (ii) a clear separation between thesurface nucleation and crystallization temperatures allowing the glassdensification to take place to completion at temperatures in between,(iv) the onset of surface crystallization following soon after thecompletion of densification providing a crystallized network thatprevents further viscous deformation.

The sintered glass-ceramics can be said to have a two-levelmicrostructure, a cellular network of crystals on the scale of the priorglass particle dimensions (2-5 μm), forming the first level within whichare formed discrete crystals of sub-micron to 1-2 μm size dispersed inthe residual glassy phase A typical microstructure is shown in FIG. 4.This unique dual microstructure is believed responsible for the highflexural strength of these sintered glass-ceramics. Furthermore, byminor additions of Li₂ O or B₂ O₃, it is possible to control the form ofcordierite that is formed and thus to control the thermal expansioncoefficient and dielectric constant within certain limits.

The glass-ceramic of the β-spodumene type as well as of the cordieritetype described above can be used for other applications besides themultilayer substrates. Such applications could be based on one or moreof their properties such as their easy sinterability, low thermalexpansion coefficients, low dielectric constant and high flexuralstrength. While some of these compositions cannot be bulk crystallized,other such as composition #1 of Table II can be used in this condition.

Also, it is to be understood that as used herein and in the claims theterm "alpha-cordierite glass" is defined as and limited to (1) aprecursor for "alpha-cordierite glass ceramics", and (2) formed from abatch consisting of, by weight

    ______________________________________                                        SiO.sub.2                                                                              45 to 55%     P.sub.2 O.sub.5                                                                         0 to 3%                                      Al.sub.2 O.sub.3                                                                       18 to 25%     TiO.sub.2 0 to 2.5%                                    MgO      18 to 25%     SnO.sub.2 0 to 2.5%                                    ZnO       0 to 2%      ZrO.sub.2 0 to 2.5%                                    ______________________________________                                    

conversely, as used herein and in the claims, the term "alpha-cordieriteglass ceramic" is defined as and limited to a glass ceramic structurecoalesced and crystallized from "alpha-cordierite glasses" to an articlehaving a microstructure of a pervasive network of 2 to 5 μm crystallitesof alpha-cordierite and clinoenstatite within the interstices of thenetwork occupied by residual glass having dispersed therein discretesecondary 1 to 2 μm crystallites of clinoenstatite and additionalcordierite phase.

Multilayer Substrate Fabrication

The glasses of the β-spodumene type and the cordierite type describedpreviously can be used to fabricate multilayer glass-ceramic substratescontaining co-sintered conductor patterns of gold, silver or copper. Thesubstrate fabrication involves the following steps:

Step 1:

The cullet of the chosen glass is ground to average particle sizes inthe range of 2 to 7 μm. The grinding can be done in two stages--apreliminary dry or wet grinding to--400 mesh particle size followed byfurther grinding with suitable organic binders and solvents until theaverage particle size is reduced to lie between 2 to 7 μm and a castableslurry or slip is obtained. A single stage prolonged grinding of culletin the medium of the binder and solvent until the desired particle sizeis obtained can also be used. In the latter case, a filtering step maybe needed to remove oversized particles. By way of example, a suitablebinder is poly-vinyl butaryl resin with a plasticizer such asdioctophthalate or dibutyl phthalate. Other suitable polymers arepolyvinyl formal, polyvinyl chloride, polyvinyl acetate or certainacrylic resins. The purposes of adding an easily evaporable solvent suchas methanol is (i) to initially dissolve the binder so as to enable itto coat the individual glass particles, and (ii) to adjust the rheologyof the slip or slurry for good castability.

Step 2:

The slip or slurry prepared as in Step 1 is cast, in accordance withconventional techniques, into thin green sheets preferably by adoctor-blading technique.

Step 3:

The cast sheets are blanked to the required dimensions in a blankingtool and via holes are punched in them in the required configuration.

Step 4:

Metallizing paste of gold, silver or copper is extruded into the viaholes in the individual sheets by screen printing method.

Step 5:

The required conductor patterns are screen printed onto the individualgreen sheets of Step 4.

Step 6:

A plurality of sheets prepared as in Step 5 are laminated together inregistry in a laminating press.

The temperature and pressure employed for lamination should be such asto cause (i) the individual green sheets to bond to each other to yielda monolithic green substrate, (ii) to cause the green ceramic tosufficiently flow and enclose the conductor patterns.

Step 7:

Firing the ceramic to the sintering temperature to accomplish binderremoval, sintering of the glass particles and their concurrentconversion to glass-ceramics by crystallization, and the sintering ofthe metal particles in the conductor patterns to dense metal lines andvias. The particular glass-ceramic composition chosen should be one thathas an optimum sintering temperature between 50°-150° C. below themelting point of the conductor metal employed.

During the firing cycle, the organic binders begin to come off at 300°C. and the binder removal is essentially complete before appreciableglass-to-glass sintering has occurred. The sintering proceeds accordingto the mechanism previously outlined and results in the conversion ofglass to glass ceramic state in which the crystalline phases formedoccupy greater than 80% of the body by volume. The holding time at thesintering temperature can vary from 1 to 5 hours. The body is thencooled at a conrolled rate not to exceed 4° C./minute to at least about400° C. after which faster cooling rates may be used.

The critical factors governing the fabrication of a multilayer substrateto close dimensional and distortional tolerances are the following;

(i) Complete and controlled removal of organic binders during the firingcycle piror to appreciable glass-to-glass coalescence. A slow heatingrate of 1° C. to 2° C. is essential for ensuring a controlled binderremoval rate.

(ii) The crystallization of glass during the sintering process whicharrests the tendency of the glass to deform by viscous flow.

(iii) The matching of the shrinkages of the conductor pattern and theglass-ceramic. Shrinkage of the metal paste is governed by factors suchas average particle size and size distribution, particle loading in thepaste and the lamination pressure. The firing shrinkage of theglass-ceramic can also be manipulated within certain limits by varyingthe binder-to-glass ratio in the green sheets and the laminationpressure.

It is noted that despite the large disparity in the thermal expansioncoefficients of gold, silver and copper, on the one hand, and theglass-ceramics of this invention, on the other, structural integrity ofthe substrate is preserved because of (i) the high degree of ductilityof these metals and (ii) good bonding between the metal and theglass-ceramic. The latter could be enhanced by the additions of suitableglass frits or other bonding aids to the metallizing paste.

When using copper as the metallizing paste, the firing of the substratehas to be done in non-oxidizing atmospheres. For this reason, organicbinders employed for green sheet fabrication should be capable of beingevaporated off in such atmospheres at reasonable temperatures.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand detail may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A glass ceramic article with the internal massthereof comprising:a homogenous distribution of a microstructure of apervasive continuous connected network of 2 to 5 μm crystallitesselected from the group consisting of (A) β-spodumene with theinterstices of said network thereof occupied by residual glass havingdispersed therein discrete secondary 1 to 2 μm crystallites of lithiummetasilicate and (B) alphacordierite with the interstices thereofoccupied by residual glass having dispersed therein secondary 1 to 2 μmcrystallites of clinoestatite, with said article being substantiallynon-porous throughout the volume thereof; and an electrical conductorpattern embedded in said article having terminal portions terminating atat least one surface of said article for electrical connection thereto.2. The article of claim 1 wherein said pattern comprises a metalselected from the group of copper, silver, gold and alloys thereof. 3.The article of claim 1 including an integrated circuit semiconductorchip of silicon mounted in electrical connection to a plurality of saidterminal portions for interconnection to said pattern, with said chipand said glass-ceramic having substantially the same coefficients ofthermal expansion.
 4. The article of claim 3 wherein said patterncomprises a metal selected from the group of copper, silver, gold andalloys thereof.
 5. A ceramic article with the internal mass thereofcomprising:a homogenous distribution of a microstructure of a pervasivecontinuous connected network of 2 to 5μm crystallites selected from thegroup consisting of (A) β-spodumene with the interstices of said networkthereof occupied by residual glass having dispersed therein discretesecondary 1 to 2 μm crystallites of lithium metasilicate and (B)alphacordierite with the interstices of said network thereof occupied byresidual glass having dispersed therein secondary 1 to 2 μm crystallitesof clinoenstatite, with said article being substantially non-porousthroughout the volume thereof; and at least two spaced andinterconnected levels of electrical conductor patterns embedded in saidarticle and having terminal portions extending to at least one surfaceof said article for electrical connection to an external electricalcircuit.
 6. The article of claim 3 wherein said pattern comprises ametal selected from the group of copper, silver, gold and alloysthereof.
 7. The article of claim 2 including an integrated circuitsemiconductor chip of silicon mounted in electrical connection to aplurality of said terminal portions for interconnection to saidpatterns, with said chip and said glass-ceramic having substantially thesame coefficients of thermal expansion.
 8. The article of claim 7wherein said pattern comprises a metal selected from the group ofcopper, silver, gold and alloys thereof.